The present invention generally relates to the stabilization of the active region of a magnetic sensor for the prevention of Barkhausen noise. More specifically, the invention relates to control of the domains at the ends of the sensing layer without reductions in signal amplitude, while preventing side-reading, thereby enhancing the operation of magnetoresistive sensors.
In the most general terms, a magnetic sensor has an active region containing a sensing layer. The sensing layer is typically a soft ferromagnetic material that responds to changes in magnetic flux. The presence of magnetic flux causes the domains within the soft magnetic material to align with the magnetic field. Therefore, it is desirable for the domains in the sensing layer to have the freedom to rotate with changes in direction and density of magnetic flux.
The changes in magnetic flux are detected through the use of an electric current passing through the sensing layer. As the sensing layer responds to changes in the magnetic flux, the electrical properties of the material in the sensing layer also change. This change is measured and converted into data signals.
Magnetic sensors are used to read data stored in the form of magnetic flux on magnetic media material, for example hard discs. The magnetic flux is stored in organized regions such that the hard disc is divided into tracks, which are subdivided into bit fields. The magnetic media material within each bit field is magnetized with a particular anisotropy. It is the changes in the directionality of the magnetic flux between bit fields that is detected by the magnetic sensor and consequently converted into data.
As a magnetic sensor reads the magnetic flux from a disc, it is critical that the sensor read only one bit field at a particular moment in time. The detection of additional magnetic flux from adjacent tracks or adjacent bit fields leads to errors in the data. Therefore, magnetic sensors commonly include shields to isolate the active region of the sensor from the adjacent bit fields within a track. Magnetic sensors rely on other methods to avoid reading signals from adjacent tracks.
In order to prevent side-reading, conventional magnetic media are usually organized to create guard bands where no flux is stored, in between adjacent tracks, thereby preventing the magnetic flux from reaching the sensor's active region. The use of guard bands limits track density. As demand for higher storage density increases, it is desirable to either reduce or eliminate the need for empty guard band spaces. Therefore, when developing magnetic sensor designs for use with high density media, it is becoming more necessary to consider the ability of magnetic sensors to prevent side-reading.
Another consideration for magnetic sensor design is proper biasing for sensor operation. Magnetoresistive (MR) or giant magnetoresistive (GMR) sensors utilize an MR or GMR element to read magnetically encoded information from a magnetic medium, such as a disc, by detecting magnetic flux stored on the magnetic medium. The MR element, comprising at least one MR layer, has magnetoresistive properties and generates an output voltage when a sense current flows through the MR layer. An MR sensor must contain both longitudinal bias and transverse bias to maintain the sensor in its optimal operating range so that it can properly detect the magnetic flux, while a GMR sensor may not need the transverse bias field, requiring only the longitudinal bias field. The longitudinal bias field serves to suppress Barkhausen noise due to formation of a large number of magnetic domains in the ferromagnetic layer, thereby obtaining a smooth and low-noise resistance change in response to the magnetic flux from the magnetic media. The dual biasing is established through various combinations of exchange or magnetostatic biasing schemes.
MR or GMR elements can “fracture” into multiple magnetic domains when they are exposed to an external magnetic field. To maximize the stability and output of the MR or GMR sensor, it is desirable to maintain the MR element in a single domain state. Three methods for maintaining the MR or GMR element in a single domain state are magnetostatic coupling, ferromagnetic exchange coupling and anti-ferromagnetic exchange coupling. Magnetostatic coupling is accomplished by positioning a permanent magnet adjacent to the MR or GMR element. Exchange coupling is accomplished by depositing a ferromagnetic or anti-ferromagnetic layer adjacent to the MR layer so that one of the magnetic lattices of the magnetic layer couples with the magnetic lattice of the MR layer to preserve the single domain state of the sensor.
Different stabilization schemes have been employed in prior art sensor structures to provide longitudinal bias for the control of the domain structure at the ends of the MR layer or active region. One method of stabilization is uniform pinning across the entire active region of the sensor by placement of an anti-ferromagnetic layer. The uniform pinning of the active region over the whole sensor has the advantage of easy processing, but reduces sensor efficiency due to a reduction in signal amplitude because of strong pinning at the center part of the active region. The need to pin the domain structure of the MR layer, particularly at the ends, to suppress noise directly competes with the desired freedom in domain movement that is required at the central portion of the MR layer in order to achieve detectable signal amplitudes. Pinning of the MR layer using a uniform stabilization across the MR element suppresses the signal amplitude, while at the same time, the pinning strength at the ends of the MR layer may not be strong enough to control domain movement. This results in the negative combination of more noise and less signal.
Another method of stabilization is to place an anti-ferromagnetic layer at or on each end of the active region. The anti-ferromagnetic layer provides biasing to pin the ends of the MR layer thereby decreasing Barkhausen noise. This method is also referred to as exchange tab stabilization. This method seems to successfully provide biasing to only the ends of the active region while leaving the center area of the sensor unpinned and capable of large signal response. However, the pinning strength at the end of the active regions may not be large enough or strong enough to maintain a single domain within the active region. Additionally, the exchange tab stabilization is not magnetically stiff enough to prevent side reading.
The predominant type of stabilization, currently used in conventional sensors, is the incorporation of permanent magnets at the two ends of the MR element. The permanent magnets provide sufficient pinning strength to reduce the noise by controlling domain movement and to define a good track width. However, the magnetic field from the permanent magnets can penetrate into the active region of the sensor making it inactive. This negatively affects signal amplitude. This problem with permanent magnet stabilization has increased as the active region size decreases in response to higher media densities. Sensors stabilized by permanent magnets tend to be very rigid in response, as well as resulting in smaller signal amplitude.
Therefore, there is a continuing need in the art for a stabilization scheme that can provide domain control of the active region for the reduction of noise while simultaneously allowing magnetic freedom in the center of the active region for large signal amplitude while reducing side reading.